I dont know if new grout would stick to sealed grout. it might, but it might not. usually whene something is grouted over again you take a sharp tool and dig some grout out from the spaces to give it room and something to latch onto. The resin would probly work fine, but I would do a little test board-slap some tiles and grout on it, seal it and then resin it just to make sure the sealant didnt cause some reaction with the resin and cloud it or something.

If I'm not mistaken, I think Home Depot carries a sealer remover. Check w/them and Lowes, as well as doing what WACKY suggests. There's also a tool for digging out grout.

Technically, you are not supposed to grout over existing grout. I did a lot of research on this at one time....(you can probably learn a lot about it if you visit some tiling sites/forums).
However, they say that if the grout is new enough, you can possibly have some success with grouting over top. But, I believe from what I've read, this is if it is not sealed. I think, from what I understand, the sealer may prevent the new grout from bonding to the old.
You are supposed to remove at least 3/4 of it first.
It's a pain, I know. I had to re-grout someone elses mistake on a backslash last year and it was not a fun time!

make sure if she uses resin she uses one that is okay for exterior use...i learned the hard way that the envirotex lite they sell at michaels is not for sunlight...my fault for not reading first