Transfer really is not a big deal in Boston for someone that age - South Station is no Penn Station or Grand Central Station, but timing will need to be taken into consideration, picking a commuter train to get her into South Station in time for her outbound train to PA. there are only 13 rail tracks at south station and they are all at the same location so she will not have to really go anywhere to get to the outbound train. The travel time into Boston from Middleboro is 1 hour and the fair is $10 in advance ( purchased at a station vending machine ) or $13 cash on the train ( exact change is requested ). I do not know if there is a vending machine at Middleboro, I have not found anything that says there is. If there is no way to purchase the ticket at the boarding station they may not charge the $3 cash fee on the train.
Kevin