I know single mums who have really really appreciated their friends holding the fort in situations not dissimilar to this.
They don't take their friends for granted, they don't do this because their irresponsible or tarts or whatever.
They do it because sometimes as a single parent, you just cannot take the strain of being accountable all the time. Sometimes to keep your sanity and the safety valves in check, you HAVE to let things fall off the radar.
These boys are lucky he's made the judgement to put them with you, you are lucky to be able to contribute to your friend's life, and he is lucky that you are so competent.
When we are all old and grey, we will want to think about all the lovely things we have done - for other people and for ourselves. Your doing the first bit, so I would say you MUST make a promise to do the second bit too...
May not be reading it right but...