Yeah. Cast another vote for the honesty party here... if she's honest with everybody, no harm no foul.
That said, if she's dating just so her friends won't hassle her about being single though, my not even remotely humble opinion is she needs to grow a backbone (and probably some self-esteem). Whether or not she dates at all should be based on what
she wants and needs, not on what everybody else expects of her.
Technically, she's not committing adultery if no one involved is married.
As to whether open marriages are adulterous, well that really depends on who you talk to. By the strictest definition, if
adultery is sex between a married person and someone not their spouse, then yes, open marriages are technically adulterous. However, since adultery usually involves cheating, the argument can be made that the real problem is dishonesty, not necessarily extramarital sex or relationships.
One's real-life mileage may vary, of course.