No Client letter from Government company for H1B RFE


My employer has filed for H1B this year and got an RFE to provide client letter (Employer-Employee relationship).
I'm currently working in EVC (Employer - Vendor - Client) model. Here the client is State Government firm. The client says they cannot provide letter on their letter head as it's against their policies. The employer says providing client letter would be strong case. I'm sure this is the case with all federal government agencies and though H1B still got approved working for them without having client letter.
Please suggest what are the other alternatives for getting this H1B approved.

I'm very worried. Please advise.

Thanks in advance!