If you don’t know if they’re using or not, I’d give them the benefit of the doubt and pray for them.
But if they’re talking about new developments in medicine, you could chime in and bring up an article. The National Institutes of Health have said that the birth control pill is carcinogenic (so google them and check out their site) and BBC did an article about a study that proves that NFP is as effective as the pill. Also, if you have access to the Harvard Guide to Women’s Health, it has some statistics that prove that NFP is accurate (though it is obviously non-Catholic and will encourage couples to use condoms w/NFP to “prevent frustration”.
In this case you’d say, “Did you know that (insert source here) said that NFP is really effective (or birth control is proving dangerous)?” But don’t sound like you’re telling them what to do, just that you find these kinds of issues interesting. Then pray that they take it to heart.