From experience -
Just be yourself. She’ll already be on high-alert for deception so … just be you.
Behave how you normally behave with him. After all, he’s just a male equivalent of a female friend, right?
She will make her own mind up regardless and if you’re “not being you” then she’ll pick up on that and think there’s something to be suspicious of.
If she’s insecure anyway then nothing you do or say will stop her being insecure, so just be you.
If needs be, have a quiet word with her to put her mind at ease that you think of him as a male equivalent of your female friends / you think of him like your little brother (even if he’s older than you) / the idea of sex with him is just eeewwww / why would he parade another woman around in front of you if there was anything going on between you / you understand her concerns but they are truly unfounded / he seems really happy with her and that makes you happy for him (that he's happy).
Get that white elephant out of the room before you try to befriend her
If the topic of your singleness crops up, ask her if she knows any single guys she can introduce you to!
Accept that the dynamic between you and your friend will change.
If you’re not comfortable with that, have a think about why that is.