I once attended a wedding where the bride's father and both of the groom's parents had died. Not only were there empty chairs at the top table, they were mentioned heavily in speeches - and their photographs were solemnly paraded up the length of the aisle. Both ways.
The marriage didn't last, for various reasons, but afterwards, the groom confided in us all that the whole honouring dead parents thing hadn't been his idea. He'd simply wanted to wear his Dad's cufflinks and carry one of his Mum's hankies in his pocket. At the time of the parading of photographs, everyone who had attended from our group of friends - and we'd all grown up knowing his parents, and had supported him when they'd died, years before he'd met the bride - was more than a bit "what the...?!" about it all, but... we assumed that he'd either suggested it, or simply gone along with it to make the bride happy.
When he married for the second time, there were mentions of his parents in the speeches, and presumably he wore and carried his mementos of them both, but no photographs or empty chairs. It felt a more cohesive wedding, because we could all celebrate a union of two people (ie, hope and life) rather than feeling sad at the loss of their loved ones/people we'd grown up knowing. His second bride (who has both parents still living) laid her bouquet upon her grandparents grave after the wedding, too, which again - I think is appropriate and lovely. She and my friend had a few private moments there, their photographer discretely snapped a photograph of them looking sombre as they did so, that was that.
OP, honour your mum and your partner's friend in your own ways. Don't let your future-MIL or anyone else tell you how to do so.