One thing I note in your friend's script is that it sets the firewall to drop all incoming connections
before allowing port 22 (ssh) connections. If something goes wrong along the way on this, you'll need console access to get back in :/ I'd recommend setting the
-P INPUT DROP last tbh
As you've noted, this is just INPUT filters, your one is just NAT/IPMASQ. So it's like comparing apples and oranges really.
Looking just at your rules, I don't see anything untoward there, it appears that you are causing the proxy to accept traffic arriving on port 80 and redirecting it to 5.196.130.245.
An external customer connecting to the address would not be able to detect the redirect and would see all their connections as going to and coming back from the address of your proxy.
The webserver would see all the connections as coming from the proxy server address, unless the customer is setting "X-Forwarded-For" headers in their requests (not all that uncommon to find) which would be passed along to the webserver and quite possibly included in it's logs.
If you have a little more background of what your end goal is, we might be able to offer more advice
I would recommend also adding a FORWARD rule to DROP anything not for that port arriving from the external interface, as you've turned on IP forwarding and at present your setup would happily forward anything anyone asks it to. It's note really serious but could exacerbate any existing security issue into a full exploit.