Amen to using HC-110 as a one-shot developer. I'd do this: go buy a fresh bottle of HC-110. Even though it lasts very well, your friend's concentrate could have gone bad. FP4 and HP5 are typically quite happy in this developer.
As to your photos: could it be that the light was not the best? It looks to be heavily overcast in the shots, which could cause the shots to lack any punch (i.e. "gray"). It's a delicate balance: a little overcast is great, but too much is too much. Try shooting during the "magic hours" on a sunny day and see what you get. In my experience, HC-110 is excellent for handling highlights. I like dilution H (see http://www.covingtoninnovations.com/hc110/index.html). Rate it at ISO 60 and develop for about 9 to 9.5 minutes.