What is an SDA file?

Open it in Open Office, and select File menu -> Save As. There you will be presented with a series of other formats, including Microsoft Office formats you can save as.

http://www.fileinfo.com/extension/sda

Sadly while you distribute it, you'll just have to indicate those formats are supported. Omnigraffle & Visio I hear work well together, but I can't find any indication if they support OpenOffice's formats, and they are more widely used. Maybe you'll have better luck. Is your intention just to distribute among people who might have OpenOffice? If so, then you will be fine.

I have a flowchart an intern made for me.

If you just need to show the flowchart in, say, a Word document, and it does not need high resolution, maybe you could just take a screenshot of it while it is being displayed in Open Office, and paste that screenshot into a page in Word. You can capture a selected portion of the screen to the clipboard by pressing Control-Command-Shift-4 and then dragging and releasing the mouse cross-hair cursor. To instead save a screenshot to a file on the Desktop, omit the Control key.