Absolutely. Most good eating apples are grown on crabapple stock. They are malleable like a Japanese maple and will graft very easily. I have much better success at approach grafting.
But it's very doable!!
If he's grafting a sucker from the base this might be below the union (if it's grafted) so therefore he might not have a crabapple any more? If
@Silentrunning is OK with that then all well and good.
FWIW Crabapples backbud profusely post chop - even if there are no visible buds - they are basically flowering elms. Here's one I thread grafted a back branch onto this past season - it curves from the right hand bottom branch to the back of the apex. I did it before bud swell so I could pass the branch through without taking off the buds.
. It looks to have taken but I'm leaving it on til I see a more noticeable difference between the entry and exit branches in terms of width. I chopped this tree initially and regrew the whole 'twisty' half of the trunk. Personally I'd chop and regrow from there - making sure it's 3 r 4 inches above the union. Do you know if your tree is grafted
@Silentrunning ? A pic might help.