Give her more time, usually the instinct to rear a lamb is so strong that a bereaved ewe will take pretty much anything.
It's a great shame that the neighbour took the first lamb back; perhaps a lesson to us all that it's worth buying lambs bought for fostering, then this can't happen. (Around here it's just a tenner. Mind, around here, no-one would ever dream of asking for a lamb back once given, no matter if it had been paid for or not.)
Anyhoo, you are where you are. Most likely, since she isn't trying to kill the lamb now, she will come around to it once it smells of her good and proper. Perhaps a few days, maybe as long as a week. If you possibly can hold the ewe long enough and often enough for the lamb to get all its milk from her (so it all smells of her and isn't mixed up with other milk smells), that will help. But obviously not to the point of letting the lamb starve.