Do they actually want a sort of 'marks out of ten, is this any good?' response, or can you just stick to 'how to make it better'?
I am on a writers' forum where people critique each other's work and honestly, it is not that difficult to say things in a positive way.
Say what the strengths are as well as the weaknesses, no matter how small the strengths are. And don't say 'this sucks because' but 'you could strengthen this by....'
One of my most useful critting friends, who has actually made an enormous difference to my writing, manages to hardly ever phrase things negatively. She says things like 'I would like to know more about this character. What are his motivations?' rather than 'This character is 2D and you've hardly bothered to develop him at all', and 'I'd like your narrator's voice to be less distanced and more immediate' rather than 'You're doing so much telling-instead-of-showing here I can't identify with her at all.'
It's only when I put her comments side-by-side with those of blunter people that I realise actually they're telling me the same thing.
I think you have to work on the assumption that the friend wants to improve and will find the negatives in what you're saying - don't worry that he might miss them. If he does that's his problem.
Even if he HAS asked you for a no-holds-barred honest assessment, rather than saying 'This is a pile of crap, you'll never be a writer' you can say 'This will probably take a lot of drafts to get it where it needs to be for submission. I recommend [insert name of appropriate beginners' creative writing book] which has some really good stuff on plotting and characterisation.'