Well - MPLS is usually a business service provided from the ISP which may be rather expensive, and afaik ISP limited, meaning you have to use the same ISP across the entire MPLS. SD-WAN simply requires an Internet Connection, whether you use AT&T on one end, T-Mobile on the other, and so on.
I believe the terms are not comparable, you can still use SD-WAN over MPLS in a special setup.
MPLS is a dedicated service with high availability and SLA, which mostly does not use the usual ISP nodes to get connected.
If your customer is insisting on a single MPLS link, versus a dual ISP Link with SDWAN, then it depends on the situation. At some locations/countries, the internet might not be as stable as required, even not resilient as required, hence an MPLS link might seem to be more efficient.
I believe you should study the situation more before advising your customer.
yes you are right and with MPLS you need to connect multiple of them in a WAN at brances or campuses hence they are very expensive to deploy but with SD WAN you can be able to provide security across WAN just from a single console hence they reduce operational cost