Don't believe everything she may be telling you. Maybe she's just telling you the boyfriend can't perform when he really can and she just wants a piece on the side.
Then you're next to be lied about and lied to.
Do you really need to get laid that bad ? And a co-worker is the ultimate NO-NO !! That's why many places forbid or frown upon co-worker dating.
If you do feel the need to engage in sexual relations, let her know upfront, NO STRINGS ATTACHED ! She won't go for that. It's all a female game. Always has been always will be. Don't play along.