You can't charge him for the extra hours you needed because you were inexperienced. As with everything the first time you do something it'll take much more time than the second time. The third will be faster still etc. etc.
If I were you I wouldn't charge him anything. Tell him you appreciate him giving you the opportunity to do this and that the only thing you would like is him to spread the word that you are available for photography jobs. But only if he really likes the result and thinks the quality of your work is good enough so he feels comfortable recommending you.
And then I would end by remarking with the right amount of subtlety
that if of course he would feel compelled to make some kind of contribution towards the cost of the lens you purchased specifically for the job that then it would seem impolite to refuse such a friendly gesture.
I wouldn't be surprised if you would get more out of this job with this approach than you had hoped for.
When in doubt..... Press the shutter.