I'm not clear why you think this is a good strategy. Are you suggesting that he should create multiple sites simply in the hope that one of them might - by luck - happen to get a high ranking? That doesn't seem likely. And would you optimise the sites for different keywords, or for the same? Either way, it's hard to see the benefit.
You said that the existing domain name is just a "random name". Are you suggesting that it would be better if it was his company name? Or a name that includes the keywords that people might search for (especially the word "mortgage" presumably)? If so, and if you consider that important, it would be simpler to register a new domain, based on your optimal keywords, and redirect the old domain to it. His customers could still use the domain name that you say he is attached to, and that name would retain the benefits of any incoming links that it has gained. There would be no benefit if registering multiple domains for that purpose.
You should also be aware that mortgages and loans is one of the most competitive businesses for search engine rankings. Frankly, juggling with domain names is not going to put him ahead of the competition. What he needs is a unique selling proposition - something to differentiate himself from the thousands of other companies in that field. That could be as simple as adding the name of the city in which he operates (assuming he is aiming for a local market). Once you know what that unique proposition is, you will be able to optimise the site in that direction.
To sum up, I suggest you tell your friend: (i) Don't over-obsess with the domain name; a well-chosen name can provide some SEO benefits, but it is just one of many signals that the search engines use to decide ranking; and (ii) Before he even starts to think about search engines, he must find a way of making his business stand out from the crowd; simply optimising a site for words like "mortgage" and "loan" is not likely to be a recipe for success.