He can attend meetings and inspections as a business visitor, but he cannot manage the plant's day-to-day operations or supervise its workforce from US soil without work authorisation. Extended visits also reduce or interrupt the qualifying year abroad, delaying the petition further.
Visits that look like work cost twice
A business visitor may negotiate, consult, attend meetings and inspect premises; directing employees, setting schedules and approving purchases at the plant are productive work that requires status. Hypothetical example: a Lake Country manager spends three weeks a month at the Washington facility during the integration, describing the trips as consultations; on the immigration side those weeks may be treated as US employment, which does not count toward his year abroad and can break its continuity, and on the admission side an officer who sees repeated long stays may refuse entry. The better sequence is to appoint an interim US plant manager from existing staff, have the Lake Country manager give direction remotely from Canada, and limit US visits to short, documented meetings until the petition is approved.
Once L-1A status is granted he may take up the role fully. The delay is measured in months; the alternative risks are measured in a refused petition and a border record.