If they both want, I dont see any reason why they shouldnt have a threesome, however I would suggest they talk about it properly first, which they obviously havent done. Talk it through, make sure you're both happy with it, set up some ground rules, etc etc.

__________________
"When I was 4 years old, they tried to test my IQ.
They showed me this picture of 3 oranges and a
pear. They asked me which one was different and
did not belong. They taught me different was
wrong."
- Ani DiFranco -