Their relationship status is their business, and I'd assume you value your buddy's judgment on whether or not this will work for him.
Rental agreements can work both ways. If she goes postal, it's possible she can report you as a renter not paying taxes on rental income (assuming nothing here). Then at the very lease you'll owe a lot of cash you may or may not have been intending to pay.
Our roommate situation works out well because he does pretty much have plenty of private space of his own. I may have a different opinion if he was sitting next to me on the couch ever single night. Is your current roommates room separated from yours enough to insulate you from sexytime (or them from yours)?
You could allow her to move in only temporarily so she could look for her own place to live, and use that time frame as a trial period. I'd be hesitant to call it a trial period to either of them so that there isn't that point where it's evident that they realize you don't like her or don't want her living with you. It'd be more of a, "hey, you've been living here for a month and I think you're cool. Instead of finding an apartment, how about you stay here with [roommate]?"
You may have figured this out already, but define what's communal household goods. Do you all buy toilet paper together? Cleaning supplies?
What about parking? Assuming you have a townhouse with a two stall garage which you consume both stalls... He parks behind the GTO. Where will she park her car?
Define chore responsibilities, or at least a way to alternate tasks aimed at maintaining common areas (Garbage, Dishes, etc.)
I figure a lot of this you've already figured out with your buddy, but a second roommate compounds this issue. I can you two having a solid enough relationship where you just make it work and don't have to be especially planful about it...but that's likely to change.
To conclude, I'd do it. But when you make the decision, be very up front about your expectations (a renters agreement) and limitations and be quick to mitigate early issues.
