I have never heard of this actually happening but if you gave somebody a lotto ticket which ended up winning a load of money as a gift/tip, would you expect them to buy you more than a pint? BTW I'm talking outside of the area of client/escort relationships!
Also the reverse question if somebody gave you that ticket, would you look after them?
I think 15% of the winning total with tax paid is about right e.g. if the win was 3 Million, the amount given should be 450K with the tax paid by the doner to give a clear 15%!