Most guild leaders don't want the bad rep that comes with unsolicited poaching from other guilds. The advice from Gracus seems pretty grown up: "speak to Guild leader and let them deal with their own.".
I'd only add, why not send a nice & simple mail to the inviter (Nice - because it may be a child / Simple - because English may not be the first language) and copy in their guild leader. You can explain why it is bad practice, how it breaks the rules and say that you will report them if you become aware of it again.
This just seems like a nice thing to do because the person inviting may not be aware of how others feel & they may not understand that aspect of the rules.
If you do see it again, report it and send a screenshot of the request and of having made the inviter and their guild leader aware. It's nice and quantifiable for support to deal with then.