If the kiddush is in shul, there should be a siddur around. Otherwise, he can ask someone who does know it baal peh to say it (without Hashem’s name) and he repeats after him. If the other person ate and needs to say a brachah acharonah, he can say it out loud, and your friend can listen and answer amen. They should both have in mind that one is helping the other discharge his obligation.
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