Roth IRA on Fidelity received a IRS Form 990-T. What is this for?

Form 990-T is for Unrelated Business Taxable Income, and Fidelity happens to have a nice help page about the form explains exactly why.

From their page, this is probably what's generating it:

UBTI tends to be generated by the following types of investments:

Limited partnerships (LPs)—businesses owned by more than one person, with limited liability to the owners for business debt

Master limited partnerships (MLPs)—a type of LP that is publicly traded and often found in the energy sector

Fidelity in this case is filling out the form on your behalf, and paying the tax from your IRA (or your family member's IRA, in this case), so it should mostly be painless on your side.

If you don't fully understand what to do at this point, follow up with a tax professional.

There are many different reasons for form 990-T, and without seeing it is hard to guess what it was for.
It could even be that some organization is telling you that they already paid taxes which you will get reimbursed as you had it in an IRA Roth:

IRAs and other tax-exempt shareholders in a RIC or REIT. If you are an IRA or other tax-exempt shareholder that is invested in a RIC or a REIT and file Form 990-T only to obtain a refund of income tax paid on undistributed long-term capital gains, follow steps above under Claim for Refund (including special instructions for IRA trustees); check the applicable box in item H at the top of Form 990-T; and attach Copy B of Form 2439, Notice to Shareholder of Undistributed Long-Term Capital Gains.

Check the IRS website which type it exactly is: https://www.irs.gov/instructions/i990t