I second what has been said - it could be anything, so try not to worry. I found a lump two days ago on one of my cats that I was certain had to be cancerous - nope, totally normal kitty mole.
Your friend trusts you to take care of the cat, so I wouldn't be inclined to call her (only because she is so far away) until you know what's going on. You aren't doing anything that wouldn't be routine loco parentis stuff, so I wouldn't call her - if it was ear mites, you'd just take her to the vet without ringing your friend first.
IF it is something complicated, you will have a much better idea of what to say after you've been to the vet, as you will know about treatment options, severity, etc. and you will have something to say because you will have factual information to relay.
Good luck, and please let us know how it goes.