Hello, my name is Julio Herrera. I will suggest you a 1.75" nut width is a good starting point for a Tele with a wide neck. For a Telecaster-style guitar with a wider neck, the width of the nut and the string spacing are the critical measurements to consider. While there isn't a "standard" wide neck size for Telecasters, you can look at a few options that are commonly used in the guitar community:
Common Neck Widths:

Standard Telecaster Neck Width:
Nut Width: 1.650" (42 mm)
String Spacing at Bridge: Typically 2-1/16" (52.4 mm)

Wide Neck Options:
Nut Width: 1.75" (44.45 mm) or even 1.80" (45.72 mm)
String Spacing at Bridge: 2-3/16" (55.6 mm) or custom

Suggestions for a Wide Neck Tele:

Nut Width of 1.75" (44.45 mm): This is a common "wide neck" size and offers more room for finger placement, which might be what your client is looking for.
Custom Neck: Some luthiers or manufacturers offer custom necks that can go even wider. Warmoth, for example, offers a "Wide" option with a 1.75" nut width that could be a good fit.
Fretboard Radius: Consider pairing the wider nut with a flatter fretboard radius (e.g., 12" or 16") for a more modern, comfortable playing feel, particularly for chord work and bending.

@julioherrera That is great feedback Julio, thanks so much. 

I’ll have a word with my client and so just how wide he thinks wide is for him. I think you are right also about the radius, I was thinking of doing a 12”

We are only just beginning the planning stage yet so there is much discussion to have. 

He is only ordering it because he loves the guitar I’ve just made for my last client. I’ll upload some pictures here later when I’ve done a photo shoot. 

Thanks for your help. 

Boo

@boo welcome!