Here is clip showing Holly Vincent on stage, seems to be from 1981, about two years after their relationship.
On a side note, I think it was her who made Mark turn to Schecters some months later, when he felt he needed a new guitar for the bigger stages and tours.
In an interview from late 1979 he said that his girlfriend has a Schecter Strat that is much better than his Fender, and that he should get a better guitar, too.
In the video - and in most pictures I found in the web - she plays this nice Schecter with the flamed body, but with a Fender neck. No idea when the neck was exchanged, or what neck it had when Mark played it.