My question is does this analysis seem valid, and should I do anything
about this? I'm afraid of ignoring it.
If you don't use Telerik components you can ignore this but it is a good thing that you are reading the logs. One thing you can do is report the attack to the IP block owner (= 'webhost'), in this case: Microsoft Corporation: ARIN link. Then maybe the client will get a slap on the wrist or their account will be terminated.
But this could become a full-time occupation: on average I witness thousands of attacks on my servers every day. It is automated attacks that may span the whole IPv4 space. So quite likely, you were not personally targeted. It probably is someone scanning a large IP range and exploiting whatever flaws they will find (fishing expedition).
It is very possible that the server in question was hacked (for example due to a weak SSH password) and is now used to hack other machines, and that the client is a legitimate person/entity who has no idea of what's going on...
I hope that your server has some defenses like a properly-configured firewall and an IDS of some sort, for example a fail2ban-like solution (I prefer CSF+LFD). Right now your SSH port may be under attack too, or whatever port is exposed to the Internet (IMAP, SMTP...). HTTP is not the only service that deserves attention.
It should be noted that many companies (or security researchers) probe the Internet en masse for legitimate research/statistical purposes, so a lot of probes may look malicious to the untrained eye but aren't.
In this particular case I am inclined to think this was a malicious attempt.
On the other hand, some research outfits (eg Shoddan) also scan for machines that are potentially vulnerable. If they find worrying results they might report on that with headlines like: "20% of IOT devices have a default password" or: "50% of IP cameras still running unpatched firmware". And you will read about it on specialized/mainstream sites about computing.
That still doesn't mean they were trying to hack into your server. They scratched the surface.