TOPIC: Keen ???

This calls for a little help from the Cambridge Dictionarie, Which gives the following entries for "keen":

Keen (EAGER)
adjective
very interested, eager or wanting (to do) something very much:
There were very keen to start work as soon as possible.
Joan wanted to go to a movie but I wasn't keen (=I didn't want to go).
She's a keen tennis player
She's keen on ( playing ) tennis.
UK my son's mad keen on cycling.
He's rather keen on a girl in his school (=he is very attracted to her).

Keen ( Strong)
adjective
1 extreme or very strong:
Many people are taking a keen interest (= a very great interest ) in the result of the vote.
2 very good or well developed:
a keen sense of smell.

As you see above, "keen" has two rather different meanings, so you're both right. You are right if you're thinking of the meaning of strong,very good. Your friend is right if he is thinking of the meaning eager.
Please note that "keen" is used more often in British English than American English. To my AmE ears, I would have throught "a keen swimmer" meant a good swimmer, but according to the Cambridge example above (in the first entry), that would not be the usual meaning.
However, if we run a concordance check (www.edict.com.hk/scripts/WConcApp.dll) on "keen" we find that the word seems to be used frequently with both meanings (of being good at something ot liking something). I suggest that you run a concordance yourself and draw your own conclusion!