Well mucus can occur in a snake without it being infection if your humidity is too low. Their natural enviornment is more humid then the air we have in our homes so if the humidity gets way too low they produce more mucus in order to keep their lungs moist and working well. Ask your friend who was watching him about where he kept the humidity level. Try putting him in an tub with a lot of holes and bring him into the bathroom with you when you take a shower and put him close enough where he/she can inhale the moisture comming up from the shower for a good 10 to 15 minutes.

I was just at the vet yesturday for two of my younger BPs and he gave me a whole long explanation of how to thoroughly check your BPs for infection before you bring them in:
Open the mouth of your BP and look at the two vains that run across the top of the mouth, if they are spotty looking and have red dots on there then you know that they are sick and need to get to the vet asap. When a snake gets a respitory infection it travels to the blood rather quickly and they get mouthrot (blood infection). This kills them quickly so don't waste any time going if that shows up.

However if there are no spots they still could have a normal respitory infection that has not hit the blood yet, this is the ideal time to catch it because it's nowhere near as deadly to the snake yet, so if you do suspect that he has one take him to the vet now so it doesn't to that point. Check to see if they have smeared mucus on the wall of their tubs/cage or what my vet sudjested was pour some water onto their head near their vents to see if mucus starts running out of their vents or nose, then wipe it off with a paper towel to see if it was indead mucus and not just water. Also sit down and take an hour or so to just watch the snake and see how their doing, check to see if they are star-gazing/mouth breathing or if they are alert. I like to sit the tub/cage in front of the tv to see if they respond to the flashing lights and just watch their reactions to everything, its time consuming but taking that short amount of time to check on their health is worth it. If anything comes up or your just still worried about it take them for the visit. It's not a cheap visit but it's worth it if you really think that your snakes are sick. My trip was around $200 with meds for two snakes, but well worth the cost to ensure your little guys help.

Good luck =]