If his goal is to maximize monthly income for the remainder of his life, he should take the monthly pension. If the pension is adjusted for inflation, he can reasonably take $3650/mo gross income (SS+Pension+4% rule from 401k). If the pension is not adjusted for inflation, I'd limit monthly gross income to $3500 so that some of the 401k can make up future reduction in the buying power of the pension.

On the other hand, if he comfortable with gross monthly income no higher than $3150/mo (using 4% rule on 401k + lump sum) and is trying to maximize his estate for beneficiaries, the right choice depends on how long he will live and future market returns (both unknowable). Since the market is likely to outperform the pension in the long run unless the pension is inflation adjusted and a shorter retirement would always favor a lump sum, I'd recommend taking the lump sum if the pension is not inflation adjusted and his gross income requirements are below $3150/mo.