document says use high 5 consecutive years in last 10.
ee accrued a benefit, quit, and now returns after 9 years. since they have a vested benefit, all service is restored.
going forward what is average comp? just the current year if that is greater than the previous hi 5??

Assuming the plan document is silent, I think you can do anything reasonable that resolves the doubt in favor of the employee, for example:

1. Ignore the break for purposes of average comp.
2. Treat him as two people and add the accrued benefits together.

I'd probably do #1.

What you cannot do is adopt a procedure which reduces the accrued benefit.

I also would lean toward suggestion 1. IMHO, suggestion 2 is not appropriate, unless it is clear in the document. If the document is like most, it will be silent on this, and Andy's comment about "reasonable" sounds good, remembering to "resolve in the participant's favor".

This assumes you have carefully reviewed the language of FAE. I have seen many which specify something like "highest 5 consecutive years in which the participant earned a YOS, out of the last 10 consecutive years in which the participant earned a YOS."