R6 trouble

A friend of mine ran into some trouble 2 nights ago.We were out for a ride and on the way home he tried to upshift to 4th gear and it was like his bike hit neutral.Now his bike won't stay in gear and is hard to get into gear.He will be riding and it jumps out of gear and when she tries to click it back into gear it jumps out.He was told by a mechanic that it sounds like the shift spring is worn.He told him to order the spring kit and he'll do the work.Does this sound like a shift spring problem or could it be worst.Maybe gears 1-3 gone?

Any help?

When you are shifting gears or when it simply slips out of gear into a "false neutral" it seems most the time the shift springs are the cause, however a gearbox has a millions little things going on in it so it is hard to pin that as the problem without really taking a look. He could also have bent the shifting fork if he does clutchless shifting.

He's going to drain the oil to see if there are any metal parts in it.If not i guess it would be the shift spring.If there are metal parts could be worn gears(dogs)bad news.Hopefully it's just the shift spring that needs to be replaced.
I was also considering installing a shift kit on my '03 YZF 600R,sometimes going to second is rough.How much is the kit and how big of a job to install it?
Thanks.