Be fair. In a monogamous relationship, some "friends" can be threatening. She may see love and sexual attraction, deep intimacy etc (all of which might actually even be a part of your friendship that you've reconciled over the years as "not going to happen") and she may wonder could she compete with that history if she had to?
Be honest. She probably couldn't. If she doesn't have anything to worry about it's because you and your friend are not interested in having that kind of relationship. That should be reassuring enough but it is still completely outside her control.
If she's been cheated on enough in the past, she may be seeing omens everywhere and trying to "learn from her mistakes" by being hypervigilant.
She may be the kind of person whose jealousy cannot be reasoned with. If that's the case, the relationship will probably suffocate under the burden of her fears, suspicions and accusations. Your friend will re-emerge with a bunch of crazy ex stories.
If the jealousy can be reasoned with, here are some suggestions on how to do so. Make eye contact with her, be friendly, indicate that you hear good things about her from your friend and that you are happy she makes your friend happy.
Example 1: Jason tells me you like Thai food, have you tried restaurant x?
Example 2: I heard you guys went to movie/concert/whatever , what did you think?
Specifically engage her about something she has an interest or expertise in. Give her a chance to dominate the conversation. Be interested.
I know from past experience that my lovers are often more threatened by old friends than other lovers because when we get together it becomes hours of "remember when" and inside jokes. This can be very alienating to the third party.