As a crew, you are only as good as the slowest or in this case the most scared pilot.
S0 yes, adapt to the situation. Wether you are a copilot or a Captain doesn't matter.
If it is safe it is ok.
However, if the behaviour is well out of the normal you should have a quiet talk with the guy/girl. The best time is after the flight, not during, and with nobody else present.
In this case you can either correct the behaviour or find out that it is a fobia. In the latter case he/she should look for professional help.
You should only be very firm in the cockpit when the situation calls for immediate action and it is not forthcomming.