Long post alert!...
Having been diagnosed as an adult, my best advice is 'persevere'. You may like to read up about AS on the National Autistic Society website (link below) which has advice about adult diagnosis.
www.autism.org.uk/
Unfortunately, my experience of seeking assessment through the NHS was a long and frustrating one and sadly, this seems to be common for adults. The NHS seemed to treat symptoms but not investigate the cause. It could be that you've got a great, clued-up GP who has prescribed Sertraline to assist your DP and will also provide an assessment referral. Or, it could be symptom-shooting. You and DP can work that out though and choose what's best.
It may be wise for DP to ask who exactly he will be referred to. What is their expertise? What is their aim? I was, at first, referred to the general mental health team. Of course, AS is not a mental health condition, it's a developmental disorder, however if this is merely the inbetween step to referral for AS assessment with appropriately qualified professionals, then that's good.
In my experience, I was next referred to consultant psychiatrist who said, when I asked him, that he knew little about AS. I was fobbed off with patronising and ignorant answers such as 'Just try harder', 'You're fine - you've got friends!' and 'People who have AS don't self-refer, it's always other people who bring them in'.
So, swat up and stand your ground. Ask questions (or have DP ask them) and keep going!
Unfortunately, I never managed to get an assessment on the NHS. I paid privately for an assessment with the Hoffmann Foundation, London (who have a self-referral option). It was a massive expense, but it's the difference between explaining why I'm different because of autism, or people assuming I'm mentally ill. It's not wrong and bad to be mentally ill, but if you're not ill and it's your personality that people think is an illness, it's right to let them know otherwise, hey?
My assessment was 3 or 4 hours long and referred to the current 'DSM' criteria. The DSM is the 'Diagnostic and Statistical Manual' for mental disorders. Now, although AS and autism are not mental health disorders, it comes under that heading in this case. Your DP could ask his assessors which set of criteria they will refer to in his assessment and it should be a recognised one such as this, or perhaps the 'Gillberg' criteria.
Next, they should be able to tell your DP what assessment method they will use to assess AS. For example, my assessment followed the 'ADOS' (Autism Diagnostic Observation Schedule).
It's also ok to ask what experience and qualifications the professionals have in the field of AS.
The Hoffmann Foundation gave me this information as a matter of course, before I had chance to ask! So, I imagine it's reasonable information to want to know.
The assessment itself was a mixture of questions and talking, with activities. There were two psychologists who made notes and later produced a report detailing how my traits matched or didn't match the DSM criteria for AS. This is the sort of outcome I would expect. During my previous attempts to have an assessment on the NHS, the consultant psychiatrist I mentioned earlier, who admitted he knew little about AS, wrote in my report that I was unlikely to have AS but if I did, I would be on the extreme mild end of the spectrum. He made no referrence to DSM criteria or any other authority on ASDs. When I was assessed in London, I was on the autism end of AS, so not mild at all! So, do be objectively critical and make sure you receive valid and respectful responses from the professionals you encounter.
Before my assessment, I chose to gather information that I thought could be useful for the assessors to gain a picture of me throughout childhood and adolescence. For me, this included school, college and work experience reports for example.
I hope your experience is a good, straight-forward one! I hope my warnings aren't daunting but rather they encourage you to persevere.