Thank you, this is very helpful. HR has confirmed that the employee's benefits were still in place during the truly, "temporary layoff" and the employee's hire date will still be his original hire date for purposes of seniority, etc.
The specific wording of the plan states the employee may enter the plan on the first day of the following month "... after his completion of three consecutive months of eligibility service...". Understanding it was a true, "temporary layoff", but needs 3 months of consecutive service, I am inclined to understand that as a) 1/15-2/15=1 month, b) 5/7-6/6=1 month, and 1 additional month is required. Is this correct?