@StevenVDM
Yes, you can. There are two ways: you can create separate WiFi mesh network with just Deco in Student room, or you can add that Deco to mesh you already have.
I am curious, why haven't you considered to buy general purpose WiFi router, some could be found cheaper than Deco M5 unit? Usually, after such router is configured, the only troubleshooting it ever needs is power it off/on, which Student room tenant can do on their own.
Finally, you must realize security implication of home network sharing. Does not matter if it is Deco or WiFi router, if this Ethernet cable from Student room is connected to your home network, all your computers, tablets and other devices on home network will be accessible from that room. If you trust the Student room tenant, it is fine. If not, consider security implications.