I think this is probably a general issue in the open source community because if I had bought a propritarian system I would have had prober support.
Not unique to open source, and extremely common in proprietary systems too. A reason I came to use open source products.
Over the last couple of decades I’ve used a ton of software both free and commercial. Proprietary systems can often be developed by smaller teams with less support coverage, but even those larger ones can still fail just the same.
I remember once a client wanted to use a system on ColdFusion (around 2003, maybe before), the software cost nearly £6k. We got no support from the start, getting fixes and updates were next to impossible unless we did them.
A few years back I installed a commercial chat software, that was around £4k and again the support was abysmal taking weeks to months before we’d even get an initial response. Ended up going to an open source XAMPP solution where support was free, a little slow but definitely there.
I also use a system called WHMCS, I’ve had great support but others complain they’ve had none. I’ve purchased add-ons costing a couple of hundred dollars and received no support but then I’ve purchased $5 add-ons and found the developer had all day to chat and help.
My early experience with closed source is what pushed me to open source, I got fed up of getting screwed and not being able to do anything about it. With open source you have the option to take charge and stuff done.
I guess what I’m saying is that just because you pay for something doesn’t guarantee it will be a sound investment with prompt and fruitful support. In fact Themeforest also state that the author does not have to support your purchase, you can also review and leave feedback for others to learn from.
It’s all about the people and how they care for their customers and business, doesn’t matter if it’s open or closed source.
Ultimately I think it’s also a case of not taking on clients you can’t handle unless you have a get out plan for when things don’t go to plan, in this case it would be factoring in some development time and billing accordingly to cover any costs.
You didn’t mention originally if it was the featured images or not, if it is then you can check here:
https://codex.www.ads-software.com/Post_Thumbnails
Hope you get this fixed up for your client.
Take care.