Participants in Sprint Retrospective Meeting

Hello,

The Scrum Guide is pretty clear :

https://www.scrumguides.org/scrum-guide.html#events-retro

The Sprint Retrospective is an opportunity for the Scrum Team to inspect itself and create a plan for improvements to be enacted during the next Sprint.

@Oliver is correct.  Product Owner is part of the Scrum Team so they should be in attendance. If your organization chooses to have your customers serve as Product Owner for a Scrum Team, then the organization has made the decision that the customer will be part of all Scrum Team events with the expectation of complete transparency between customer and your organization. 

If you feel that having the customer as part of the Scrum Events is causing your Development Team to be less transparent, it seems to me you have identified an impediment.  Is there any way that you can solve the impediment without excluding Scrum Team members?  What if you took this to the leadership of your organization and suggested that having internal Product Owners would improve the team dynamics, their work product and possibly their productivity? The customers could be used as Stakeholders who the Product Owner can consult for defining stories and the Stakeholders are expected to be included in the Sprint Review. 

I do have a concern about one thing you said even though it doesn't directly apply to your question but could be used to help support my suggestion above. Emphasis added by me.

 In my team, the Product Owner and Customer ( Both are from third party ) are same set of people. 

To me My bolded text indicates that this is more than 1 person.  While here I will provide this from the Scrum mGuide. Again, emphasis added by me.

The Product Owner is one person, not a committee. The Product Owner may represent the desires of a committee in the Product Backlog, but those wanting to change a Product Backlog item’s priority must address the Product Owner.

Is it correct for Customer to present in Sprint Retrospective meeting and pass their comments ?

What valuable comments are they in a position to make regarding how the team inspects and adapts its process?

The Review meeting exists for this exact purpose. The PO can invite the customer and they can share their feedback with the team.

The Retrospective meeting is an internal meeting where the team attempts to find out what went well and what didn't. Having third parties like managers or customers attend the Retrospective meeting will result to reduced transparency as team members will - most likely - not share their thoughts freely in fear of exposing themselves, the team or their colleagues. As a result, this will lead to Retrospectives where "everything is fine" and no actions will be taken to fix actual problems.

Worse, the customer may start dictating the fixes that the team should implement to its process, leading to reduced autonomy and plummeting morale.