get her a PC. it’ll solve your problems, and lower your stress level 
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and you’re already sold on the idea to not use SU for mac anymore. and it’ll be cheaper than going mac. Teach her the basics of windows, teach her your workflow on a PC.
edit 3 : I realise the tone of my initial message was sharp, I just argued with an idiot. allow me to explain. I work on both, with a preference for my mac, muscle memory and all. but whenever I have to work in a team on a project, I’ll use the same as them. This summer I’ll work on a furniture project for a school, I’m giving a hand, I know they all have macs, so I’ll bring the mac mini and a screen. I have a client whose firm is on PC, when I meet them, I bring the PC. It’s easier, same machines, same language, same tools, no weird thing like you describe.
So yeah, if she had been a competent SU on mac user, she could have remained on a mac, but from what you describe, I would recommend putting her on a PC. Training will be simpler for you, as well as interoperability. Sell the mac, buy a PC, and with the extra money, buy icecream. it helps with the learning process. trust me, I teach 
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edit : if, however, you maintain her on a mac… does the problem happen only with trebuchet ? other too ? can you show us screenshots of the pc/mac difference ?
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re-edit : ok, I have a lead.
Fonts, like softwares, have different versions.
Having macosX gives you one version while installing a miscrosoft product gives you another
if you go to /System/Library/Fonts/Supplemental (menu go / go to folder) you’ll find one version, in my case, the 5.00x. it appears this is the version installed at the root, in the OS (not just for me)
if you go to ~/Library/Fonts/ you’ll find the version installed just for this user. 1.23 for me
Plus, I also found a copy in /Library/Fonts/Microsoft , a folder containing the fonts microsoft office installed in 2021 (when I installed id for a quick job). In that case, the trebuchet version was 1.26
On my PC, the Trebuchet MS installed seems to be 5.15
could it be the issue ? that she is using an older version of Trebuchet MS and windows don’t accept it ?
Did you install some microsoft office stuff on her computer ? or is she using the standard Trebuchet MS font maybe ? it’s all just speculation.
a definitive test would be to move all the current trebuchet (and trebuc) she has, and install the one from your PC (just transfer the otf or ttf file and double click on mac). if it works, then that was it. if it doesn’t work, you can always move the older versions back.