You could take St. Thomas More's tack in A Man for All Seasons:
To me [your music] seemed delightful. ... I should in fairness add that my taste in music is reputedly deplorable.
Not, of course, in those words, but something along those lines: Sorry, I'm not the reader you're looking for. Your poetry is probably delightful but it doesn't strike a chord in me (or, more neutrally: It's nice but I don't have much to say about it, sorry). Speaking as a writer of poetry, humility about the arts is a perfectly reasonable way to send me a message.
This assumes that your goal is to avoid being faced with the awkward situation of being sent poetry with the expectation of a response that you don't feel inclined to give. If your goal is to actively improve this person's poetry, you have to ask yourself a few questions first:
Are they looking for constructive criticism? Not all sharing of creative work is about that. Often, serious writers realize that the people around them aren't critics, just audiences, and don't seek feedback from them. And less serious writers may not want to go through the editing process if they've expressed what they feel and it's served its purpose. Make sure to respect that some people write for other reasons than to make refined art.
Are you qualified as a critic, or would you just give your own impressions? If the latter, will those be of use to them — is it what they were looking for? Sometimes people just want a sympathetic reader, and it's no offence to find that someone isn't. You shrug and move on.
Is it your style of poetry? I find that I'm a decent critic of poetry, but I also recognize that there are many dissimilar genres. Some are not my taste. I have no useful feedback to give to the writers of those kinds, just my blessing to go do what they enjoy.
If you still want to go the constructive criticism route, I suggest you simply check first. Ask if she would like constructive feedback on the poem. If she says no, OK. And even if she accepts, assume it's tentative — amateur writers often enthusiastically ask for feedback without knowing how it will go. Reflect on how she reacts, and if it doesn't seem appreciated, also OK: no need for a repeat trial.