If we remove the Van Allen Belt from consideration and only consider GCR exposure in cislunar space, it can be seen that based on the 7 month data of the MSL/RAD that background radiation is approximately 450 ugy (.45 mgy) per day.
For that 7 month period, yes. What did the data in that period actually look like? I'm currently doing a project investigating news reports of the ISS in the British press, and over a 20 year period there's an average that is heavily skewed by the fact that reporting on Tim Peake's mission in 2015/2016 meant that more articles appeared in the press in that period than in the rest of the 20 years put together. Large peaks and high variability skew averages badly. The cislunar radiation environment is heavily dependent on the Sun, and we know that's not a steady source, with quiescent periods and flares of widely ranging magnitudes.
This would indicate that as a base line all lunar flights should have as a minimum exposure rate of at least .45 mgy/day.
No. See above. Also consider the rain analogy. You can't conclude, based on average rainfall, that everyone going outdoors on any given day should come in with a minimum level of wetness.
If consideration is given to the fact that GCR radiation is 20 times more damaging
The source for that claim is what? And again, 20 times more damaging than what? Radiation doesn't scale that way. Few things do. Too many variables are being ignored. Another analogy: people have survived being impaled with huge objects, but other have died from a relatively tiny gunshot wound.
and shielding for GCR is best provide by hydrogenous material,
Best doesn't mean only, and in terms of spacecraft there are other considerations besides effectiveness of radiation shielding that have to be taken into account. Structural and thermal properties for starters.
then the baseline should be at least a magnitude higher.
No, for the reasons given.
If the 4 hours plus trips through the VAB are included in the assessment
Which parts of the VAB, what levels and kinds of radiation are there, and how intense were they?
Your friend is still making too many oversimplified assumptions about radiation. Jay's question is valid: these data have been published for decades, and if no actual experts in the relevant fields are finding fault with it, why does your friend think his conclusions are valid?