Tell them to take a long walk off a short pier....they are trying to dupe you.

 

We are not getting all the information here - so we can't say for sure.
450ES - gotta be 5 years old
Through water? over the tires or the exhaust?
What was the bike like before all of this?
what was the repair like?
how experienced is the friend on an atv?
If the oil was milky then it was water contaminated....
..... for me , bottom line, if I thought I had a responsibility then I would contribute to the repair.
(Then again ... I once crashed a friend's Range Rover that she loaned me while she was out of the country for a few months -- when I told her, her reply was 'crahing it was part of loaning it', and she wouldn't take a cent.)
How far did it go in Hunting season? -- how often?.. how well was it running?
How come you don't trust these 'friends'?
did you trust 'em before all of this? before it might cost?
See.... more questions than answers... and most of the questions got nothing to do with an ATV. Mebbe common sense is needed more than a mechanic.

 

I agree with tramp and those above. You need to tell this guy to take a hike. If the water crossing would have caused any problems they would have shown up earlier. And if the water was only up to the top of the tires I HIGHLY doubt enough water could have gotten in to be a real issue. Ive submerged my bikes to their handlebars. An oil change or 2 and drain the carb and tank and good to go. This guy is trying to get you to pay for his upgrades. Find a new friend. But please remember all us riders are not like this.