Their payment can be computed using the formula for amortization.
... A = P(r/12)/(1 -(1 +r/12)^(-12t)) . . . . where ...
... ... P is the principal amount of the loan, r is the annual rate, t is the number of years, A is the monthly payment.
Filling in your values, this becomes ...
... A = 150,000·(0.055/12)/(1 -(1 +.055/12)^(-360))
... A ≈ 851.68
Their monthly payment is $851.68.