I was hoping it was going to be something simple like that
But your post has prompted me to read up on this, and am very sorry I have to say it doesn't look good. It could be a high price to pay for doing someone a good turn. Apparently the problem can occur when you disconnect the jump leads. Suddenly the flat battery is disconnected from the running alternator on the donor vehicle and you get a 'Load Dump'
https://en.wikipedia.org/wiki/Load_dump which in worst case scenario can cause a high voltage spike when all your alternator power suddenly has nowhere much to go. But that depends on many factors and it seems I have been lucky to avoid it so far. Possibly due to my secondary battery bank absorbing the spike. So I have bought a new set of jump leads with surge protection (reduced to only £10 at Lidl for 3.5m x 25mm2 equivalent cable) It has a little black box in the leads which is supposed to absorb the spike - as long as you disconnect the flat battery car first. Sorry thats of no help to you but might help others reading this before they help someone out like you did. Please let us know how you get on. I really hope its something minor.