My friend is trying to say something like “through hardships to the stars, prepared for all things” or something of the like by combining “per aspera ad astra” and “omnia paratus”, but I’m pretty sure that it’s more complex for that. I took Latin in high school but I’m not sure if and where the cases might change and to what?
Figulus explained that per aspera ad astra omnia paratus does work grammatically. What you should know in addition is that the correct form of paratus depends on who is prepared:
|The one(s) prepared||Form of participle|
|Group of males or mixed group||parati|
|Group of only females||paratae|
There are neuter ones too, but they sound unlikely. If you want to express "I am prepared", then use the singular endings -us or -a as appropriate. If you want a general "we are prepared", then go with the masculine plural ending -i.
The bare accusative omnia can be used with paratus, exactly as you have done. It looks okay to me.
There are always other options, of course, if you are interested. You can say in omnia paratus or ad omnia paratus, or you can use the dative omnibus paratus. Lewis and Short cite all of these usages from classical authors.
It is a matter of taste, of course, by I prefer all of the other options to the bare infinitive, myself. Of course, if I were trying to fit the phrase into meter, I might feel differently.
You also might prefer saying omne, everything, to omnia, all things. Or not.