Is there a method to escape the & character to keep it from being used as an & variable. The code below errors out on the &body statement:
-* File emailtest.fex
SET BYDISPLAY = ON
-RUN

TABLE FILE CAR
PRINT
MODEL
COMPUTE BODY/A19 = "&body=This is the email body";
BY COUNTRY
BY CAR
ON TABLE HOLD FORMAT HTMTABLE
ON TABLE SET STYLESHEET *
TYPE=REPORT, GRID=OFF, FONT='ARIAL', SIZE=8, $
TYPE=DATA, COLUMN=COUNTRY, JavaScript='fnMail(COUNTRY CAR BODY)', $
ENDSTYLE
END
-RUN
-HTMLFORM BEGIN
<script LANGUAGE=JAVASCRIPT>
function fnMail(Email,Subject,Body)
{
window.location = 'mailto: someone@somewhere.com' + '?Subject=' + Subject + Body;}

!IBI.FIL.HOLD;
-HTMLFORM END

Have you tried using default. We use it to keep WebFOCUS from prompting when the '&' is in a report heading

-DEFAULT &S = '&S'

Try this,

SET BYDISPLAY = ON
-RUN

TABLE FILE CAR
PRINT
MODEL
COMPUTE BODY/A30 = 'body=This is the email body';
BY COUNTRY
BY CAR
ON TABLE HOLD FORMAT HTMTABLE
ON TABLE SET STYLESHEET *
TYPE=REPORT, GRID=OFF, FONT='ARIAL', SIZE=8, $
TYPE=DATA, COLUMN=COUNTRY, JavaScript='fnMail(COUNTRY CAR BODY)', $
ENDSTYLE
END
-RUN
-HTMLFORM BEGIN

<script LANGUAGE=JAVASCRIPT>
function fnMail(Email,Subject,Body)
{
alert("BODY : " + Body);
var tt='mailto: someone@somewhere.com' + '?Subject=' + Subject + '&' + Body ;
alert("tt : "+ tt);
-*window.location = 'mailto: someone@somewhere.com' + '?Subject=' + Subject + '&' + Body;}
}


!IBI.FIL.HOLD;


-HTMLFORM END
-EXIT

Simply add a concatenation character whenever there's HTMLFORM involved (by the way, I made a couple of other small changes to make the program work:

-* File emailtest.fex

-SET &ECHO=ALL;

SET BYDISPLAY = ON
-RUN

TABLE FILE CAR
PRINT
MODEL
COMPUTE BODY/A30 = '&|body=This is the email body'; NOPRINT
BY COUNTRY
BY CAR
ON TABLE HOLD AS H001 FORMAT HTMTABLE
ON TABLE SET STYLESHEET *
TYPE=REPORT, GRID=OFF, FONT='ARIAL', SIZE=8, $
TYPE=DATA, COLUMN=COUNTRY, JavaScript='fnMail(COUNTRY CAR BODY)', $
ENDSTYLE
END
-RUN

-HTMLFORM BEGIN
<SCRIPT LANGUAGE=JAVASCRIPT>
function fnMail(Email,Subject,Body)
{
window.location = 'mailto: someone@somewhere.com' + '?Subject=' + Subject + Body;}
</SCRIPT>
!IBI.FIL.H001;
-HTMLFORM END

I think Francis' idea is the quickest and the one I use most. I find that simply inserting a concatenation | symbol after the ampersand works in most cases.