I can't see any reason why a Jew, a Muslim, a Hindu, a Buddhist, a Protestant, or an Atheist can't attend a Mass if they want to. I'm not sure what the difference is between an interested spectator and a disinterested specator, but either way they'd be more than welcome.

And I'd have no worries myself as a Catholic attending a service at any other place of worship. I've done it often enough, and it's a nice change

Why make a big deal out of a communal celebration? There's a whiff of intolerance around.