Unless you would feel uncomfortable due to something like major disrespect between you (the kind where all literally slammed down phones, name called, shouting matches blah blah), I'd likely say 'sure'.

I'm sure you've managed to have a polite lunch with other people who you would have prefered not to socialize with. Sometimes one just does these things because not to would come across rude or cause additional unneeded resentments.

You say she has been trying harder with her son. Maybe her illness has forced her into realizing that life is short, she was 'wrong' on her behavior towards her child the first time around and wants to now turn over a new leaf.

Though hard on you/DH to let he past go (which is understandable), I think giving the mother a chance to show she is trying to change could be beneficial to SS. It's only like an hour or so, it gives DH a chance to fill her in on anything she needs to know for her summer...who knows him best and how to deal with the teen than Dad/you and it's been Dad/you doing it all this time. If she does not ask questions, don't volunteer suggestions though as that might not go over well.

If lunch goes well, you say thanks and hit the road. If it turns unpleasant, cut it short and leave. I'd give her a chance. Perhaps not because I really wanted to, but because SS may be upset if you all turn down the invitation. If you say 'no' you are going to have to give the teen an honest answer as to why.

Really only you know if this is a positive thing or not to have the lunch. If you think it could cause more problems, then politely refuse. But if you feel it can be harmless enough (if even undesired) and it can be pleasant for SS to see the adults (his role models) in his life interacting and getting along for a mere lunch, it might be postive.

A whole summer just the two of you? Woo-hoo! Have a great adult summer break and enjoy your free time. I also hope things go well this summer for SS and that he has a pleasant summer (well, at least as pleasant as a teen can have...they can be a handful during the teen years).

Lots of friction for 10 years...Um I would say no...I m a believer in No good deed goes unpunished....I doubt it would be important to a 17 yr old that you lunched together after all these years....Say you have other plans and thank you for the kind offer....

To be honest, since your DH simply _told_ BM he was dropping off SS for the summer without any consultation, I am amazed she is being gracious enough to offer you lunch. While I agree that as his mother she should have been more involved in the past, she may have already have made plans for her summer based on past summers, which are now in disarray. And it's nice to be asked, rather than told.
Anyway, I think you should accept the lunch invitation. Apart from anything else, if you just drop off SS and take off, since you have changed the previous arrangement and are leaving him for the whole summer, he may form the impression he is being "dumped" and having the whole summer to simmer over it, will be even more difficult when he comes home. JMO.

--"I doubt it would be important to a 17 yr old that you lunched together after all these years."--

My thinking on this one was because the teen is soon old enough to make or break his own relationships. One day not too long in the future this teen may decide he's had enough 'friction' in his young life and decide his Dad/SM is too high maintenance. This suddenly involved BM has several months to influence the teen (good/bad) and he just might decide (right/wrong) he 'sides' with his mother.

Thank you for your input, everyone. There has been a lot of friction in the past and quite frankly, there has been little to no communication between DH and BM for the last 5 years. To sit down and break bread with her would be extremely awkward for everyone involved, except perhaps BM, who has always had difficulty understanding boundaries and just doesn't get it that we don't like her at all. She has a very high opinion of herself (to put it mildly!).

As for SS17, as far as we know he doesn't know anything about this invite, and whether she tells him about it or not, I don't believe he would care one way or the other.

Colleen, the reason DH told her instead of asking is because he's just tired of always holding the bag so to speak. He knew that if he asked, she would have a list of reasons why she couldn't do it. We seriously couldn't care less about whether she had plans for this summer...if she has, then she can include her son in them. We expect she'll probably ship him off to her parents or her sisters for most of the summer anyway, so it won't matter.

As SS17's mother she has an obligation to take on the responsibility of her son when needed, and right now that's what DH needs. SS17 is aware of the reason he's being shipped off to her house. He knows how his behaviours have affected his father (and me) and he's old enough to realize that we are not "dumping" him. Whether she influences him one way or the other isn't something we're worried about. He will be 18 in October and whatever choices he makes after that will be up to him. However, we are certain that this experience will open his eyes to how good he has it here, something he doesn't appreciate at the moment.

Lady Q, You re on to something LOL You dont like her, it will be awkward, dont do it!!! Some times these things are over analyzed....SS doesnt care, done and done :)

Yes, I think you're right, dotz. We've decided to just ignore it, like we usually do with her hair-brained emails. She doesn't deserve as much thought as I've put into this. DH hasn't given it a second thought - his initial comment was "lunch in hell? I don't think so." And, for him, that was the end of the story, and so it will be the end for me also.