"Her faith in us is strong."
Translation:Haar geloof in ons is sterk.
Unfortunately, you can't.
When we're talking about someone being 'trouw' to someone, it is about being loyal.
She may be loyal to you but that is not the same as her having faith in you. :)
Here are some examples with sentences that contain 'trouw zijn', by the way:
- Ik ben trouw aan het vaderland. - I am loyal to the fatherland.
- Ik blijf je altijd trouw - I will always be loyal to you.