Certainly posters who've never worked in a bar or not for a very long time or not a busy one!
I have (admitted very long ago but well remember), mum worked in bars most of her life and dd works in one now - glass collecting!
Employer should have given training on lifting AND provided appropriate equipment and enough staff! Given how busy it sounds. There are trolleys, glass washers designed for busy bars where the racks rotate up so you don't have to bend right down to lift them out and so on. Even trays for collecting in less busy places which have across the back straps that distribute the weight.
But sounds like boss a tight arse too and wouldn't get this stuff.
Also other employee shouldn't have been given the job if not up to it. Age is irrelevant I know 60 year olds who are/would be fine doing this kind of work, others that couldn't even cope just with the constant standing. If she has a back issue she's not fit for the role. At the very LEAST she should be doing the other work needed, keeping tables and bar/counters clean, changing optics, drying glasses (I would not be surprised if your glass cleaner is cheap shit that doesn't dry them), keeping snacks, stirrers etc topped up. There's PLENTY to do in a bar no time for standing Blethering!
It's 10 hours, you have the bar experience AND other work experience on your cv - I say get another job - and tell the boss why. People are reluctant to do this because of refs but I've done it when I've left rotten jobs and it's never been a problem. I actually took great delight once in telling an AWFUL woman boss (to the point of abusive and sexually harassing the male staff) of EXACTLY why I was leaving in her office in front of the regional manager (I wasn't planning to but she made a snide comment as I handed over my resignation letter - bad move)
As pps have said you MUST take care of your back. Mine is done in due to a couple of daft accidents (resulting in slipped discs, sciatica which is inoperable. Then a car accident which caused another condition - which I was more vulnerable to due to slipped discs) I'm in pain EVERY day, can't stay in one position for more than about 20 mins even lying and if it's really bad can't trust my legs to work). I'm only 45.
My mum has mainly done bar or retail work and done the heavy lifting but always did it correctly and refused if too heavy or wrong equipment. She's retired now (late 60's) but because she looked after her back she doesn't have a problem there.
Bloody hell! And the tips should be getting shared too! You're contributing to the customers good experience in your graft, not on if you're too busy elsewhere doing stuff she won't so she can serve.
Not AT ALL surprised they can't keep staff.