If you would introduce a new food to a hypersensitive child, you would do it with a very small amount first, and if that does not cause negative reaction, you would increase the amount the next days.
The same with FMT .
Go slow.
Small amount of feces in a small amount of water the first time you try, and if that works, increase the amount gradually, day after day.
If the first one give bad results, change the donor.
Try to at least find a donor of the same blood type, just in case it is of importance.