I am unsure how to act in this situation. So please give me advice.

My girlfriend asked me if it would be okay for me if she met with an old friend of hers. She added some shit about him breaking up with his girlfriend and that he started to text her a lot since then.

Of course this is probably some form of shit test so I kept calm and just said that I'll think about it and changed the topic.

Telling her that I don't want her to meet him for obvious reasons would be honest but a sign of weekness. Allowing her to meet him is basically allowing another man to hit on my girlfriend.

Apart from everything I just wrote the relationship is otherwise healthy, sex is regular and I am in good shape.

So what do I do? Just shrug it off in a stoic manner or enforcing boundaries that I don't want her to meet other men alone?