Can an employer make matching contributions to a solo 401k?

I have a client who wants to match 100% of their deferrals is this allowed? 

If allowed by the plan document.

I am seeing elsewhere that for solo's it is only non electives are you sure that this is accurate? 

Then you are a reading a document that doesn't allow for matching contributions. A Solo-K is simply a marketing term describing a 401(k) plan that covers only the owner. But often times it just has has limited document choices about that is offered in the document and may not offer everything that might be allowable under the code.

Also, if it's only the owner, what difference does it make if it's matching or non elective? The 415 and 404 limits will impact both the same.

I see okay so then an employer can match 100% of the deferrals of the employee? or does the 25% rule still apply here in that they can't match more then 25% of their contribution since they can't profit share more then 25% of their compensation? 

You still need to comply with 415 limits and the overall employer deduction limit is going to be capped at 25% of pay under 404. Just because you have a match instead of a profit sharing doesn't get you around the limits. That said if you have a REQUIRED match in the document it might force you over the 404 limit on deductible contributions and force a nondeductible contribution subject to an excise tax which might be one reason some "solo-k" documents don't allow matching.

For example say you have a document that that requires you match deferrals dollar for dollar with no cap. And have the following set of facts -

W-2 owner (only employee) pay $20,000.

W-2 401(k) contribution $10,000

Employer required match $10,000

The match is clearly required by the document and clear not in violation of 415 limit so it has to be made.

But compensation is only $20,000 so the 25% deductible limit on employer contributions is 25% or $5,000.

Therefore $5,000 of the match would be required but non-deductible and also subject to a 10% excise tax.

Not an ideal result and not a set a facts you'd be likely to see but just an extreme example of what could be allowable and still get you a bad result.