The founder lives with the consequences of every decision long after the conversation ends. You don't. That asymmetry is the whole point: your job is to make their decision better, not to make it for them.
Strong mentors are often strong operators, and operators are used to deciding. The temptation is to say “if I were you, I'd do X” and move on. It feels helpful. It isn't: a founder who executes your plan without owning it will abandon it at the first obstacle, and will never learn how to make the next call alone.
So push hard on the reasoning. Surface the options they haven't considered, the risks they're underweighting, the evidence they're missing. Disagree openly when you disagree. Then step back. If they choose a path you wouldn't have chosen, help them execute it as well as possible, and agree on what would tell them it isn't working.