Free MCE seats for BAEC elites is a published benefit; free MCE seats for Cathay Pacific elites is not. If one is entitled to free MCE seats because of status, the benefit extends to traveling companions in the same PNR. No exclusion for Basic Economy is listed:

"AAdvantage® status member access


"Who gets complimentary Main Cabin Extra seats?


"AAdvantage Executive Platinum®, AAdvantage Platinum Pro® and AAdvantage Platinum® members


"oneworld® Sapphire and Emerald members with elite status on Alaska Airlines, British Airways, Iberia, Finnair, Qatar Airways or Japan Airlines


"All listed members are allowed complimentary access for up to 8 traveling companions on the same reservation."

However, the AA page describing Basic Economy says that only AAdvantage elites get to select a seat for free on Basic Economy fares, so there is a conflict between what the MCE page says, and what the BE page says. From the BE page:

"You can choose a specific seat at any time for a fee. AAdvantage® members can choose complimentary seats based on their status."

If so inclined, your BAEC Silver traveling companion could contact AA, cite the language on AA's MCE page, and try to snag MCE seating for free (for both of you, if you are both traveling on the same PNR).