I would use my sewing machine with a stretch buttonhole.

Nan, I agree that making the buttonhole using your sewing machine would be best. The yarn threads of the sweater will not be held tight enough to hold properly with hand stitching without puckering or twisting. Just my opinion.
Angel

If you are using an embroider design button hole or any other for that matter use stickey on back but I personaly use it on the front I would use spray on reg interfacing bucause stickey on front will be hard to take off and might show

I would use sticky on back and heavy solvy on top. If I have occassion to sew together knitwear, I use solvy in the seams to hold together til I am done then spritz it away.....

If you could poke your finger through the sweater then use yarn to do a hand blanket stitch around the opening. Using yarn doesn't cut the fibers like thread does. It would have been ideal to have knitted in the buttonholes when it was constructed. You can pull apart the yarn ply to have less bulk.

Thanks to all who answered. I have decided not to do the buttonhole after all as the girl to whom it belongs does not want it ruined and I am a big chicken! Thanks cuties. Nan