This dudes a Russian national with a cold war attitude. There is no convincing them as there views a set in stone. They are the opposite of people here in the US who think Russia is the devil itself.

With that being said, they are right in some respects. Companies really want to know who your friends are as it is very useful for advertising. Google messager and contacts keep a close eye on who you are talking to and when. The same goes for WhatsApp and any other messager that doesn't protect metadata. I think the metadata is actually more valuable than the raw text. It also is concerning that Mobile carriers have complete access to everything about you including location, communication logs, communication metadata, internet traffic, SS number, and so much more. I don't believe Signal to be harmful and it is definitely better than SMS and phone. Signal also has the advantage of being more reliable and popular than many apps. For instance, Signal calls are way better than Simplex Chat. Signal is also good for contacts you normally would use with regular phone service. You would use there phone number anyway so you aren't losing anything.

The TL;DR is that Signal does not protect metadata which can be a problem. However, if you are using Google or Apple messaging it doesn't really matter