Allying with Muslims in the mid 14th century

During the mid 1300s, what would be the attitude in the European community of a Western European ruler allying with a Muslim ruler in order to settle a dispute among Christians? Would this invoke a crusade or excommunication?
 
The Catalans' alliance with Turkish raider beyliks earned them an excommunication and made them the target of several crusade plans. OTOH, the Pope didn't manage to organize a proper Crusade against them, just lesser actions. And everyone kind of hated those guys anyway.
 
So like, say if an Italian merchant republic did it people would be pissed but it’s not like people would be Deus vulting?

Not likely, no. Though I suppose it would depend on the exact nature of the "alliance". If its a hard-core, back each other to the hilt sort of thing you're probably going to get some scorn, but co-operation to settle some specific issues/against a particular state (provided its not one the Pope is overly fond of) probably won't get you in too much trouble.
 
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