First of all, make sure that your friend's car meets the criteria to be used for the driving test.
If it does, then you will need to be insured on that car to be able to drive it. Your friend has comprehensive insurance, not you. So you will either have to become a named driver on his policy or take out a learner driver policy.
Unfortunately, you cannot get short term learner insurance for 1 day only; the minimum is 4 weeks. Your best bet would be to take a few driving lessons with an approved driving instructor and then use his car for the driving test (driving instructors will already have insurance to cover learners).
And yes, if you don't have adequate insurance on the test day, you will not be allowed to do the practical test.