Instant message question?

i had my friends aim screen name for about 8 months and last time we talked was last week and we talked and then today i said hi (and my friends name is not keegan)and all sudden his account someone said like this"this is keegans mom and if you do stop ill report you to police and said i was 16 on ipod or computer"i got a bit freaked from that,so i texted him and said hey and then i dont know who replyed but said "stop texting"then the person said who was my friends # on my contacts said "your last warning"and i called to see if its him the whole time and when i called a girl voice picked up and said hello so i hung up so i deleted that number and aim account to prevent myself in trouble.tomorrow should i ask my friend Gouping at school and tell him about it?