Singlespeed & Fixed Gear - Problem: Cranks "Slipping"

My friend got his Dawes SST-AL today and it was riding fine for awhile, then all of a sudden he starts up for a ride when the light turns green and as his cranks are at 12 & 6 (right foot, dominant foot at 12) he tries to pedal, the pedal at 12 goes straight down. he said it feels like it just slips down to 6. Im guessing its either the bottombracket or his lockring isnt tightened. any insights?

Lockring it probably is, and or a stripped hub.

tighten down the cog & lockring. If that's not it, he's completely stripped his square taper BB. Or, he's stripped his hub.

It is prob the lockring but the hub might be stripped now too.

Sometimes you just need to regrease the theads on the lockring and cog. If its too dry it'll just slip and spin.

cog and lockring arent tightened. happened to me a few months ago. dont ride until you get them properly tightened or youll make matters worse

go tighten your lockring NOW. i stripped my stock hub on the kilo 2 weeks after i got it because it kept "Slipping" and i didnt know what that meant~

If it is the lockring, it could slip both forwards and backwards depending on where the cog is. If he just recently skidded and/or stopped abruptly with his pedals, then the cog would have moved backwards so when he would start at the stoplight, it would slip forwards.