Tropical fruit tree fertilizers here generally have something like a 4-1-6 ratio. I do use P (eg, triple super phosphate) if I'm trying to encourage root development, but on the mango that's best in the fall, after harvest and when the tree is naturally laying out new roots.
But, as shot pointed out, phosphite is not really used to provide P. It's an antifungal.
As far as nitrogen goes, mangoes need very little. You can give your mango trees nitrogen to bolster production, as long as you provide enough calcium (a lot of it). Otherwise, you'll get jelly seed. You probably want to time the application to be after flowering as well (most likely not an issue in socal where you guys get hundreds of chill hours per year).
The other problem with N here is that a mango tree can go from 3 gallon pot to 15 feet in 4 years -- not a good thing unless you're wanting a single specimen shade tree and don't really care about hurricanes destroying your trees every few years.
Most other tropical fruit trees benefit from N.
Ideally the ratio is 1 to 5 or 1 to 6 on P to K (not 1 to 1 as this product is). Diluting doesn't change the ratio. Also want to make sure it has a good micronutrient package.
I’ve been using 0-10-10 products for around 2 decades with great success. You can just use it for your P-K and combine it with other products to get a ratio you want.
Since he has the fertilizer already, he shouldn’t just throw it away. Mangos and other fruit actually use some Nitrogen in blooming and fruit formation. Usually some residual complete fertilizer from slow release(summer fertilizer) or saved up in the wood of the trees is enough for the tree when combined with 0-10-10 or similar to stimulate good blooms and initial fruit set.
Simon