I wouldn't suggest it in general. There's way too many sketchy places that could hide malware inside an app. I.e. They claim to offer paid apps for free/pirated, but you wind up with a compromised phone. The only time I'd make an exception is if you're getting the app directly from a known developer. In your case, if the developer was willing to send it to you because it's not on Play. I know some sites like Adobe have legacy apps listed on their website for download, even though the apps may no longer be officially supported.
If in doubt, don't do it. If not in doubt, research the sources and options you have, even checking the Amazon app store. Amazon may have the app when Google doesn't and is a safe alternative. Don't just download an apk from some place simply because it popped up as the first one or two results when you googled it.