It’s a tough one.
When I got married 26 years ago my dad’s GF was there. He was a widow (mum died) and I didn’t want him upset or uncomfortable and his wellbeing is important to me. She sat next to him, supported him, and showed me her dress to make sure I was ok with it. To be fair to her she actually helped me out a bit with my wedding, and calmed my dad’s nerves at speaking publicly etc. When I picked out a dress, my dad didn’t like it and thought it was really expensive (Duchess satin Ritva Westenius £2K dress when my wedding in total cost 7k including dress - sorry just had to drop that in as I’m still reeling at how great that dress was) and I tried it on upset, came out and she cried at how beautiful it was and went outside and convinced him to buy it for me. She’s not my mum, never tried to be, and was in my corner. Why would I treat her so disrespectfully?
On the day it didn’t bother me that she was there, or at the top table, and she held his forearm gently as he spoke about my mother and we all cried.
However, during the evening she did excuse herself as one of my brothers was rude to her, and she didn’t want to ruin my day. I also got stick from a few different directions about inviting her, letting her sit at the top table etc. Apparently I was disrespectful to my mums memory.
Would I do it again? Yes, because I am a grown up. She took nothing away from anyone. I’m not a Bridezilla. Other people’s comfort and happiness at our wedding celebration mattered to me. Take note modern day brides! The only people who tarnished my day were those who threw stink bombs into the circle and said “ just saying”.
The mature thing is for the bride and partner to say, look it’s about <bride> day, and she wants <op> there, as she’s a friend, and part of her life. Of course everyone’s sad her mum isn’t there, but it doesn’t take anything away.
There’s no reason why the OP can’t go and keep a low key, and be there to support the dad when he has a moment away from the day and wish the bride well.