It's a two way street. You are fair, so the customer should be fair. It was someone else who made the bad hose, and left the hose off. It's ok if you want to keep a customer coming back and are ok with working for free once in awhile. I know people can be difficult on both sides. This is for a third parties car, if you cover for the other guy's bad work, there is some satisfaction in that help for another worker, maybe once or twice, but not ongoing. Another problem is you are working and driving across town for more nitrogen etc, in addition. The other guy which is a middle man it seems, isn't doing that. It would be really bad if he pads your price and makes the customer pay more than you got. I guess, no expert, but I had similar things for people who wanted me to test and make things for free like my time didn't count. Theirs always counted.
